In connection with the completion of oil and gas wells, it is frequently necessary to utilize plugs, packers, or other sealing tools in both open and cased boreholes. The walls of the well or casing are plugged or packed from time to time for a number of reasons. For example, a section of the well may be packed off so pressure can be applied to a particular section of the well, such as when fracturing a hydrocarbon bearing formation, while protecting the remainder of the well from the applied pressure.
A sealing element on a tool, such as a packer or a plug, typically has an initial diameter to allow the tool to be run into the well. The sealing element is then expanded to a radially larger size to seal in the wellbore. Such a tool typically consists of a mandrel about which other portions of the tool are assembled. For example, a fixed gage ring is attached to the lower end of the mandrel, and a push ring slidably surrounds the upper end of the mandrel. If desired, a slip assembly can be used on the mandrel to lock the tool longitudinally in place in the well. In any event, a sealing element is disposed on the mandrel between the fixed gage ring and the push ring. When compressed between the rings, the sealing element creates a seal between the mandrel and the surrounding wall, thereby preventing fluid flow past the tool.
Typically, when the tool is set, the mandrel is held in place and force is applied to the push ring. The push ring moves towards one end of the mandrel, causing the various parts of the tool's sealing element to be longitudinally compressed but radially expanded. As the push ring slides down the mandrel, the sealing element is compressed longitudinally. Most sealing elements are an elastomeric material, such as rubber. When compressed longitudinally, the sealing element tends to then expand radially to form a seal with the well or casing wall.
Unfortunately, the sealing element's expansion may not be limited to only being radially outward. Instead, due to the forces applied during expansion or the force of the pressurized fluid upon the sealing element, the sealing element may extrude longitudinally along the tool through the spaces between the fixed gage ring and the well wall and/or between the push ring and the well wall. Due to the unwanted possibility of extrusion, anti-extrusion rings can be used to prevent the sealing element from extruding beyond the fixed gage ring or push ring, which would cause the tool to fail. Such anti-extrusion rings are employed along the mandrel between the ends of the sealing element and any push or gage rings or other components on the tool.
The anti-extrusion rings may be an elastomeric material, such as nylon, that may not seal as well as the sealing element. However, the anti-extrusion rings may deform enough to prevent the sealing element form extruding to the point of failure. In some instances, metal materials, such as lead, copper, or steel, have been used as well for anti-extrusion rings.
One common structure used for an anti-extrusion device is a cup. The cup fits against the end of the sealing element so that the element's end fits partially in the interior of the cup. The outer bottom of the cup fits against a gage ring or push ring. As the sealing element expands, the cup opens by splaying into a petal like arrangement. The expanded cup or petals tend to limit the longitudinal expansion of the sealing element. To increase the efficiency of the anti-extrusion device, multiple layers of cups may overlay one another so that any gaps, such as between the petals of a split cup, will be overlapped by the adjacent cup.
For example, a downhole tool 10 having a cup-style anti-extrusion ring 20 according to the prior art is shown in FIG. 1A. The downhole tool 10 is an open-hole packer having a mandrel 12 on which are disposed a hydraulic piston 14 and an end ring 16. A sealing element 18 is disposed between a push ring 15 of the piston 14 and the end ring 16. When moved by the piston 14, the push ring 15 compresses the sealing element 18 longitudinally against the end ring 16, which causes the sealing element 18 to expand out radially.
Cup-style rings 20 are provided on the ends of the sealing element 18 at the push and end rings 15, 16. These Cup-style rings 20 help prevent over-extrusion of the sealing element 18. For example, FIG. 1B depicts a side cut away view of a prior art anti-extrusion ring 20 after the sealing element 18 has been expanded against the casing C and the mandrel 12 to seal the annular area A, thereby preventing fluid flow past the tool 10. As the sealing element 18 expands radially outward, the leading edge 26 of the sheath 22 of the prior art anti-extrusion ring 20 is also pushed radially outward to contact the casing C.
Further details of the cup-style ring 20 are provided in cross-section in FIG. 1C. This ring 20 is a petal-style foldback ring having a number of petals 22 connected at their proximal ends by a neck 24 and separate by gaps or slots 26 toward their distal ends. During use, the petal-style ring 20 opens by splaying into a petal-like arrangement as discussed above.
Another cup-style ring 30 shown in FIG. 1D lacks petals and does not splay open into a petal-like arrangement. Instead, this ring 30 has a widened sidewall 32 that fits partially along the outside surface of the sealing element (18) and the element's end. The sidewall 32 extends over the end of the sealing element (18) from a wider neck 34 that fits at the mandrel (12) and push or end ring (15, 16) of the packer (10). The distal end of the sidewall 32 has an integrally formed lip 36, which is rounded in shape. As can be particularly seen, the thickness of the sidewall 32 lessens from the wider neck 34 to the lip 36.
Unfortunately, cups may be easily damaged as they are run into a well. Additionally, they may be damaged during setting when they are radially expanded into sealing contact with the well or after the element and cups are set because the tool may move longitudinally due to varying forces acting on the tool in the wellbore. Therefore, a need exists for an anti-extrusion device that tends to limit or prevent any damage to the anti-extrusion device during run-in and use downhole.